1. Field of the Invention
This invention relates to a belt driven continuously variable transmission and especially relates to a belt driven continuously variable transmission suitable for using in an automatic variable transmission to be mounted on an automobile more particularly, it relates to a pressure regulating mechanism section generating a axial force to correspond to a transmission torque.
2. Description of the Prior Art
Generally, this V-belt type continuously variable transmission (CVT) has a primary pulley and a second pulley each of which is made up of a movable sheave and a stationary sheave. Metallic belt is wound around both of these pulleys. A transmission shift done at a required moment by moving the movable sheave with a hydraulic piston.
Therefore, the continuously variable transmission, where a hydraulic pressure is used, needs an oil pump and a hydraulic pressure passage. This structure makes the device not only large and complicated but also unfavorable for the transmission efficiency and for the belt endurance because the structure requires the belt holding force more than needed, which furthermore, makes it impossible to transmit due to belt holding force becoming short in supply when the hydraulic pressure decreases some reason.
The applicant of the present invention has proposed that a belt driven continuously variable transmission 30', as shown in FIG. 13, having a metallic belt 33 around a primary pulley 31 and a secondary pulley 32, whose movable sheave 31b and 32b are moved axially by actuator mechanisms 35 and 36 such as ball thread mechanism, and to arrange a pressure regulating cam mechanism 34' imparting an axial force which corresponds to a transmission torque to a fixed sheave 31a. (refer to the Japanese Laid Open Patent No. 62-13853).
By the pressure regulating mechanism 34', this belt driven continuously variable transmission 30' gets the belt pushing force which is in proportion to the transmission torque, however, with reference to the shifting ratio only a constant belt holding force can be obtained.
While, in the continuously variable transmission the belt holding force varies by the shifting ratio, and when the shifting ratio gets small the required belt holding force gradually decreases even at the full opening of the throttle.
However, the continuously variable transmission 30' has the pressure regulating cam mechanism 34+ to generate a constant holding force according to the lowest speed shifting which requires the largest holding force. So at the lower speed shifting state of frequent use, a large belt holding force works. As result, a load much more than necessary works on the V-belt, the bearing and the pressure regulating cam mechanism, and causes decadance of transmission ratio, and maintenance and noise troubles.
This invention has as its purpose provision of a belt driven continuously variable transmission which is designed to have adequate belt holding force by varying axial force working on pulleys according to speed shifting ratio.